1. Field of the Invention
The invention is directed to acousto-optic devices. More particularly, this invention is a method of relieving the mechanical stress between the transducer plate and the optical medium thus substantially reducing undesirable strain-optic effects. The invention also provides a transducer plate for effecting this reduction in mechanical stress.
2. Description of the Prior Art
Acousto-optic devices generally consist of a transducer plate and a transparent optical medium through which acoustic waves generated by the transducer propagate. While the transducer is typically a piezo-electric crystal such as lithium niobate, other materials including quartz and PZT (lead-titanate-zirconate) are often used. The optical medium may be glass, such as fused silica or it may be crystalline, such as tellurium dioxide (TeO.sub.2) or thallium arsenic selenide (Tl.sub.3 AsSe.sub.3). Typically, a bond of high acoustic quality between the transducer and the optical medium can be obtained by the following process. A thin film of metal, such as indium, is deposited on the surface of the transducer and on the surface of the optical medium which are to be mated. While under mechanical pressure, the combined transducer and optical medium are heated in a vacuum oven to the melting point of the metal film and then cooled to ambient temperature. Indium requires heating to about 155.degree. C. Other types of bonding methods, including the use of an epoxy, typically require that the bonding procedure be carried out at an elevated temperature. Once the bond is established, the transducer is polished down to a thickness determined by the frequency of operation of the device. In these bonding methods, one electrode is first deposited on the optical medium. After bonding and polishing, the second electrode is deposited on the transducer.
These fabrication procedures present several problems in acousto-optic devices. Because the transducer and the optical medium have different thermal expansion coefficients, the portion of each member near the bond is under mechanical stress when the acousto-optic device is cooled from its bond formation temperature. The mechanical stress produces very undesirable strain-optic effects which distort the optical properties of the optical medium by inducing birefringence. Such an effect often renders the device useless for many applications. When the transducer is polished to a small thickness, often as small as 0.025 .mu.mm. for high frequency operations, it is subject to stress cracking due to the mechanical stress forces at the interface with the optical medium. This cracking generally renders the transducer useless.
A problem that arises with large area transducers, on the order of several square centimeters, or even with small area transducers at very high frequencies, is that of matching the electrical impedance of the transducer to that of the electrical driver. It is especially true for the materials of a very high dielectric constant that the impedance of the transducer may be so low that it becomes difficult to efficiently couple electrical power to the transducer. This problem can be largely overcome by electrically dividing the transducer into series connected mosaic elements, as taught by Weinert and deKlerk (IEEE Trans. on Sonics and Ultrasonics, SU-19, 354; July 1972). If a transducer of a given area is divided into N elements, the capacitive impedance of the transducer will be reduced by a factor of N.sup.2. This is accomplished by an electrode pattern that is deposited both under and on top of a transducer plate. These electrode patterns are electrically connected in series. Through this method, the capacitive impedance of the transducer can be modified to match the fifty ohms impedance of the electrical driver.
It is an object of this invention to provide a method for bonding a transducer plate to an optical medium so that mechanical stress at the interface of the bond is substantially eliminated.
It is also an object of this invention to provide a transducer plate for use with an optical medium. The transducer plate facilitates handling and aligning with the optical medium during bonding operations. After polishing is completed the transducer plate of this invention is reduced to a series of individual transducer elements bonded to the optical medium.